An important trend in development of semiconductor technology is scaling down of metal-oxide-semiconductor field effect transistors (MOSFETs) for improving integration level and reducing manufacturing cost. However, it is well known that short channel effects arise as the size of MOSFETs decreases. As the MOSFETs are scaled down, a gate also has a reduced effective length and actually controls fewer charges in a depletion region when a gate voltage is applied. Consequently, a threshold voltage of the MOSFETs drops with a reduced channel length.
In the MOSFETs, it may be desirable on one hand that the threshold voltage of the device is increased to suppress the short channel effects, and on the other hand that the threshold voltage of the device is decreased to reduce power consumption in a low supply voltage application, or in an application using both P-type and N-type MOSFETs.
Channel doping is a known approach of tuning the threshold voltage. However, if the threshold voltage of the device is raised by increasing the doping concentration in the channel region, mobility of carriers drops, which results in a degradation of the device performance. Moreover, ions with a high doping concentration in the channel region may neutralize ions in source/drain regions and ions in regions which adjoin the channel region, which decreases a doping concentration in the region adjacent to the channel region and increases resistance of the device.
It is proposed by Yan et al. in “Scaling the Si MOSFET: From bulk to SOI to bulk”, IEEE Trans. Elect. Dev., Vol. 39, p. 1704, July 1992, that short channel effects can be suppressed by disposing a ground plane (i.e. a grounded back gate) under a buried insulating layer in an SOI MOSFET.
However, the above SOI MOSFET comprising a grounded back gate still cannot meet the requirement of the device for the threshold voltage when the channel length of the semiconductor device is kept shrinking.
Therefore, it is still desirable that the threshold voltage of the semiconductor device is adjusted in a controllable manner without increasing the doping concentration in the channel, while the performance of the semiconductor device is not deteriorated.